1. Field of the Invention
This invention relates to the use of compressed gas propellants with plastic aerosol containers.
2. Description of the Related Art
Pressurized containers for dispensing aerosols are well known in the art, and are typically constructed of metal in order to withstand the inherent internal pressure of aerosols. However, it is desirable to provide a plastic container capable of withstanding the internal pressures generated by an aerosol because plastic has many advantages over metal. Some of these advantages include the ease and economy of manufacture, and aesthetic appeal to an end user.
The two main types of propellants used in aerosol dispensers today are liquefied gas propellants, such as hydrocarbon and hydrofluorocarbon (HFC) propellants, and compressed gas propellants, such as compressed carbon dioxide or nitrogen gas. In an aerosol dispenser using the liquefied gas-type propellant (also known as a double phase propellant), the container is loaded with the liquid product and propellant, and pressurized to a pressure approximately equal to, or slightly greater than, the vapor pressure of the propellant. Since the container is pressurized to the vapor pressure of the propellant, a majority of the propellant is liquefied. However, a small portion of the propellant will remain in gaseous form. As the product is dispensed, the pressure within the container will decrease and more of the propellant will enter the gas phase. In a compressed gas aerosol dispenser, the propellant remains in gaseous form when the container is pressurized for use.
U.S. Pat. App. 2004/0144863 to Kendrick et al. discloses the problems involved in trying to maintain a small particle size mist over the useful life of the aerosol, using a liquefied gas propellant in a metal container where a constant pressure is maintained by the reservoir of liquefied propellant. The problems are much more difficult in plastic aerosol containers using pressurized gas, as suggested in U.S. Pat. App. 2005/0242101 to Skalitzky. Pressure in the aerosol container is proportional to the amount of compressed gas propellant (unlike liquified gas propellants which maintain constant pressure) so the amount of propellant that can be used is limited. Also, compressed gas propellants permeate through plastic containers. The low usage level and permeation combine to limit the shelf life of the aerosol and may require upgraded resin or other costly bottle enhancements to have an acceptable shelf life. Finally, with compressed gas propellant, the pressure in the aerosol container drops as the product is used. This results in changing spray performance as the product is used and limits the utility of plastic aerosol containers using compressed gas propellants.
The problems with compressed gas propellants in plastic aerosol containers can be avoided by recognizing the criticality of the relationship among the particle size requirements, headspace volume and pressure, and gas permeation through the walls of the plastic aerosol container.